The present invention concerns a method for cleaning a mixture of air and vapour from volatile crude oil with recovery of the hydrocarbons.
The invention also concerns a system for use in the method.
Volatile crude oil has a high vapour pressure at normal temperatures both in summer and winter. The high vapour concentration above the crude oil entails that a considerable loss occurs owing to the evaporation in the handling of the oil, e.g. in the filling of storage tanks. It is clear that it is of extremely great importance to reduce theses losses, both for economic reasons and for working environment and safety reasons.
Therefore, a plurality of methods aiming at reducing such losses have been developed in the course of time. Thus, the Danish Patent Application 0257/83 discloses a method and a system for recovering petrol from a mixture of petrol vapour and air by absorption of the petrol in a cooled petroleum distillate having a higher boiling point range than the boiling point range of the petrol. The petroleum distillate is successively cooled by heat exchange with a cold reservoir, brought into direct contact with the mixture of petrol and vapour to absorb the petrol, transferred to a buffer tank and transferred further to a stripping means which may be a distillation column. When thus cooling condensation and absorption of the petrol vapour are combined and the amount of cooled petroleum distillate brought into contact with the mixture of petrol and air is controlled so that the petrol concentration in the petroleum distillate transferred to the buffer tank is substantially constant, excellent optimum control of the petrolum absorption process is obtained, both in average load operation as well as in peak load operation. Reference is made to the above-mentioned Danish patent application for more detailed process and apparatus features.
In spite of its advantages, such a recovery system has certain drawbacks, one of the greatest being that the system takes up rather much space. In connection with oil exploitation from drilling platforms on the open sea it would thus be an advantage if the volatile hydrocarbon vapours could be recovered already when the volatile crude oil is pumped into tankers, but the large dimensions of the system known from the Danish Patent Application 0257/83 makes it less suitable for installation on a tanker. Furthermore, it is difficult to obtain a sufficiently high effeciency with the known system when the content of methane and ethane is high.